The present invention relates to sports racquets such as tennis racquets, squash racquets, badminton racquets, and racquetball racquets, which have hollow tubular frames made of composite materials such as xe2x80x9cgraphitexe2x80x9d. High performance sports racquets have a hollow tubular wall made of graphite material. To make such racquets, an elongated tube of pre-preg, formed of uncured graphite, is placed in a mold in the desired shape of the racquet frame. A bladder placed inside the pre-preg tube is inflated, such that the pre-preg assumes the shape of the mold, and the mold is heated to cure the epoxy resin and harden the frame.
After the frame is made, holes are drilled through the opposing walls of the tubular frame to support the ends of the racquet strings. If left uncovered, the sharp edges of the string holes would cause serious string breakage problems. For such reason, composite sports racquets employ plastic grommet strips to prevent direct contact between the strings and the holes in the frame. The grommet strips ride in a stringing groove formed along the outside surface of the racquet head portion, and contain a plurality of hollow grommet pegs, which extend through the holes in the frame. When the racquet is thereafter strung, the strings exit through the hollow barrels of the grommet pegs, and bear against the grommet strip along the outside of the racquet until reaching the next string hole, in this manner avoiding direct contact with the graphite frame.
Over the years, newer, stiffer frame materials, together with advances in molding techniques, have allowed composite sports racquets to become increasingly light. Today""s graphite tubular frames as molded are very strong and very stiff, even with very thin wall thicknesses. However, when racquet string holes are subsequently drilled in the racquet, carbon fibers are broken and the frame is weakened locally. This problem is exacerbated by the fact that the string holes must have a diameter large enough not just for a string, but for a plastic grommet peg. As frame walls have become increasingly thin, the process of drilling the string holes can weaken the frame significantly, to the point where the frame is unable to support the high forces of the tensioned strings, resulting in strings pulling through the walls of the frame tube.
Commonly owned U.S. Pat. No. 6,071,203 discloses a two-piece sports racquet comprising a tubular upper frame half and a tubular lower frame half. The two racquet halves are molded separately from one another, and then bonded together, along opposed walls, using epoxy. The opposed walls of the two racquet halves are molded with half cylindrical channels which, when the two halves are joined, form the string holes of the frame, thereby eliminating the need to drill string holes and thereby weakening the frame. Also, when the two racquet halves are joined, the opposed walls become an internal wall in the frame which is oriented parallel to the direction of the string force on the frame, thereby further helping to prevent string pull-through.
While the invention disclosed in U.S. Pat. No. 6,071,203 provides a beneficial improvement in sports racquets, it would be desirable to provide a two piece-racquet having the same structural advantages, but without the need to bond two racquet halves together following molding.
In a method for making a composite sports racquet frame according to the invention, a mold is provided having first and second mold plates which, when joined, define a mold cavity in the shape of a sports racquet. A first tube of a material suitable for forming a racquet frame, such as a fiber-reinforced resin, is placed in the first mold plate. A second tube of such material is placed in the second mold plate. A plurality of pin plates, each having a plurality of pins which are preferably at least substantially parallel to one another, are secured to the first mold plate so that the pins lie on top of the first tube.
After coupling a compressed air source to each of the tubes, the mold plates are secured to one another so that the first and second tubes contact one another at least in the regions to either side of the pins. The mold is then heated, while at the same time pressurizing the tubes, to form a racquet frame in which the tubes are joined to one another at least in the regions adjacent to the pins.
Preferably, the tubes are made of a composite material having a component, such as epoxy resin or thermoplastic, which, when subjected to the heat and pressure of the mold, will flow. In this manner, during molding the contacting portions of the two tubes will fuse together to form a common, integral, internal wall.
After the molded frame has been removed from the mold plates, the pins are pulled out of the frame, the pins thereby forming string holes in the frame.
Preferably, the pin plates forming the string holes in the head portion of the racquet are disposed on the outside surface of the head portion, so that the pins project inwardly. The base of the pins, i.e., where the pin project from the pin plates, are contoured so that the string hole openings formed by the pins along the outside surface of the racquet are rounded, thereby reducing the chance of string breakage.
Preferably, the tubes, when in the mold, alternately contact one another and a pin along the entire head portion of the frame. Preferably, the ends of the tube which will form the racquet handle portion are kept separated from one another during molding, by using a removable mold plate.
Also, preferably, the inner surface of the pin plates, from which said pins project, forms part of the mold cavity, e.g., to define a string groove extending along the outside of the head portion of the frame between string holes.
For a better understanding of the invention, reference is made to the following detailed description of the preferred embodiments of the invention, taken in conjunction with the drawings accompanying the application.